1. Field of the Invention
The present invention relates to lasers and in particular to lasers using non-linear crystals to obtain shorter wavelength radiation from the longer wavelength radiation by a frequency-conversion process.
2. Related Art
Each successive node of semiconductor manufacturing requires detection of smaller defects and particles on the wafer. Therefore, yet higher power and shorter wavelength UV (ultraviolet) lasers for wafer inspection are always in demand. Because the defect or particle size is reduced, the fraction of the light reflected or scattered by that defect or particle is also typically reduced. As a result, an improved signal-to-noise ratio is required to detect smaller defects and particles. If a brighter light source is used to illuminate the defect or particle, then more photons will be scattered or reflected and the signal-to-noise ratio can be improved as long as other noise sources are controlled. Using shorter wavelengths can further improve the sensitivity to smaller defects, because the fraction of light scattered by a particle smaller than the wavelength of light increases as the wavelength decreases.
In general, semiconductor wafer inspection and metrology needs can be met by continuous wave (CW) lasers with high beam quality (e.g. with M2 close to 1, wherein M2 is the ratio of the beam parameter product of the beam to that of an ideal Gaussian beam of the same wavelength). If CW lasers of sufficient power and beam quality are not available, then the next best alternative is generally a high repetition rate laser, e.g. with a repetition rate of approximately 50 MHz or higher. Such high repetition rates are possible with mode-locked lasers (which is a type of pulsed laser). Q-switched lasers have repetition rates that are much lower (lower than 10 MHz, usually lower than 1 MHz). Generally, mode-locked lasers are capable of emitting extremely short pulses on the order of picoseconds or even femtoseconds. A mode-locked laser induces a fixed phase relationship between the modes of its resonant cavity such that interference between those modes causes the laser light to be produced as pulses.
Beam quality (e.g. as measured by M2) is important in semiconductor inspection and metrology applications because a laser beam must be focused to a small spot (or line) to detect small defects or particles and/or to measure small areas. If the beam quality is poor, then the focused spot (or line) on the wafer is not Gaussian in profile and the tails of that profile contain more energy than ideal. Those bigger tails result in at least some of the signal being collected from outside the area of interest, thereby reducing the contrast of the signal from the area of interest.
Non-linear crystals can be used to create a UV laser beam by generating a harmonic of a long wavelength beam or by mixing two laser beams of different frequencies to create a frequency equal to the sum (or difference) of the two frequencies. Because the harmonic generation and the mixing process are non-linear processes, higher incident power density typically results in a more efficient conversion process and higher output power.
However, increasing the incident laser power on a non-linear crystal can have undesirable side effects. Specifically, a high power level may change the refractive index of the crystal (photorefraction). Because the focused laser spot in the crystal has an approximately Gaussian profile, the intensity is different at different locations within the crystal. Therefore, the change in the refractive index varies with location in the crystal. This refractive index gradient in the crystal can distort the output beam, thereby resulting in astigmatism. As the quality of the output laser beam worsens, the spot or the line on the wafer generated by that beam becomes broader and thus less efficient for detecting small particles or defects. Although small amounts of astigmatism may be approximately corrected by optics placed in the beam path after the crystal, such correction will only be approximate and will only be effective if the initial astigmatism level is very low.
Another undesirable side effect of higher incident power level on the crystal is that permanent damage may occur in the crystal over time. With accumulated exposure, this damage can result in generally decreasing power intensity as well as generally increasing astigmatism. Therefore, correcting the astigmatism with optics would require frequent compensating adjustments, which would be impractical in commercial applications. Moreover, the astigmatism may also rapidly increase to the level where accurate compensation is not possible even with adjustment.
Generating a shorter output wavelength can also accelerate the degradation of the crystal because the output photons are more energetic and therefore can change characteristics of, or even permanently damage, the crystal. Thus, at shorter output wavelengths, astigmatism and other adverse beam quality and intensity effects may also increasingly occur.
The optimum power density in the non-linear crystal is a balance between maximum conversion efficiency (which usually requires as high a power density as possible), and minimizing color center formation, photorefraction, and two-photon absorption (all of which are minimized by lowering the power density) while maintaining a good beam profile.
Notably, photorefraction and two-photon absorption may cause a temporary change in optical properties, which persists for at least the duration of the incident laser pulse and, typically, for a short time thereafter. When the laser repetition rate is low, as in a Q-switched laser, there may be sufficient time between one pulse and the next for these changes to the crystal to partially or fully relax back to the original state. This relaxation may be faster if the crystal is operating at a high temperature (such as between 120-150° C., which is a typical temperature range for standard operation). Applications in the semiconductor inspection and metrology typically are better served by very high repetition rates (such as 50 MHz, 100 MHz, or higher) as can be achieved by mode-locked lasers. However, such high repetition rates typically do not allow time for the changes in crystal properties to relax substantially from one pulse to the next.
Non-linear crystals, such as CLBO (cesium lithium borate) or CBO (cesium borate), can be used to create deep UV light from the 2nd harmonic of a visible laser light input. For example, 266 nm wavelength light can be created from a 532 nm laser beam using CLBO. In another embodiment, light near 213 nm wavelength can be created by mixing, for example, 266 nm and 1064 nm wavelengths. The maximum power level at which such crystals can be operated is limited by defects and impurities in the crystals.
Impurities in a crystal or defects in its crystal lattice can degrade the lifetime of the crystal or create color centers that become locations at which changes in the crystal optical properties happen faster than elsewhere in the crystal. Thus, to the extent possible, the highest purity of starting material should be used for fabricating the crystal.
Notably, impurities, such as water, can be incorporated into a crystal during its growing process or even during normal operation (when used in an inspection system) even if not present in the starting material. These impurities can adversely impact the crystal lifetime at high power densities. Unfortunately, improving the purity of the starting material does not reduce impurities that are incorporated during operation.
One known technique for reducing or slowing deterioration in the crystal is to operate the crystal at a high temperature (typically between 120-150° C.), which generates higher energy electrons in the crystal. These higher energy electrons are able to move around more easily, thereby cancelling out some of the light-induced changes in the short term. This technique is most useful for low repetition rate lasers because there is a relatively long time interval between pulses (which allows recovery from the effects of one pulse). This high operating temperature can also help prevent absorption of water by the crystal while it is in use.
Although operating the crystal at a high temperature can provide more electrons with high enough mobility to neutralize some changes in the crystal, it also increases the energy of defect states in the crystal. Thus, the high operating temperature can slow some defect mechanisms while accelerating others. Notably, the increased temperature is less effective at reducing short-term changes in the crystal when the repetition rate is high.
Another known technique to deal with crystal damage is to use one location in the crystal for a period of time, then move to a new location before there is too much degradation of the output beam quality and/or intensity. Frequent adjustment of the location in the crystal used for frequency conversion means that for a significant fraction of the operating time, the laser is being adjusted, realigned, or is stabilizing after an adjustment. Even if the adjustment and realignment are automated, there may be times when the laser is not operating to full specification while it stabilizes after adjustment. In inspection and measurement applications in industries like the semiconductor industry where manufacturing facilities run 24 hours a day, this interruption to, or reduction in, operating time is a significant disadvantage. If the damage rate is high, even with frequent and automatic adjustments of the conversion location in the crystal, the crystal may still last only a few days or a few weeks before needing to be replaced. Such a short time interval between service events is unacceptable in the semiconductor industry.
Therefore, a need arises for a high power laser system that includes a frequency-conversion crystal (i.e. a non-linear crystal capable of generating harmonics of the fundamental laser wavelength), yet can ensure a high quality, stable laser beam and a long crystal lifetime.